The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Emissions control is an important factor in engine design and engine control. One particular combustion by-product, NOx, is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion. Rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. Reduction of NOx created in the combustion process and management of NOx in an exhaust aftertreatment system are priorities in vehicle design.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in exemplary devices known in the art within the broader category of aftertreatment devices. However, one having ordinary skill in the art will appreciate that aftertreatment devices are largely dependent upon operating conditions, such as device operating temperature driven by exhaust gas flow temperatures.
Modern engine control methods utilize diverse operating strategies to optimize combustion. Some operating strategies, optimizing combustion in terms of fuel efficiency, include lean, localized, or stratified combustion within the combustion chamber in order to reduce the fuel charge necessary to achieve the work output required of the cylinder. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust gas flow, can be greatly reduced from normal values. Such conditions can be challenging to exhaust aftertreatment strategies, since, as aforementioned, aftertreatment devices frequently require an elevated operating temperature, driven by the exhaust gas flow temperature, to operate adequately to treat NOx emissions.
Increasingly stringent emission standards require NOx aftertreatment methods, utilizing, for example, a selective catalytic reduction device (SCR). An SCR utilizes a reductant such as ammonia derived from urea injection or recovered from normal operation of a three-way catalyst device to treat NOx. Additionally, it is known to operate a diesel oxidation catalyst (DOC) upstream of the SCR in diesel applications to convert NO into NO2 preferable to treatment in the SCR. Continued improvement in exhaust aftertreatment requires accurate information regarding NOx emissions in the exhaust gas flow in order to achieve effective NOx reduction, such as dosing proper amount of urea based on monitored NOx emissions.
Aftertreatment devices such as SCRs convert NOx to nonpolluting molecules at some conversion efficiency. Conversion efficiency can be described by the flow of NOx flowing into a device versus the flow of NOx exiting the device. An aftertreatment device operating properly experiences reduced efficiency according to properties of the exhaust gas flow that affect the chemical reaction occurring in the device. For example, temperature and space velocity of the gases within an SCR device affect the efficiency of the device. These environmental factors can be monitored in the aftertreatment system, and effects of these factors upon device conversion efficiency can be estimated. Additionally, malfunctions or degraded performance caused by wear or damage can reduce the efficiency of the aftertreatment device. However, reduced efficiency can additionally occur when insufficient reductant, such as ammonia supplied by urea injection, is available on the SCR catalyst.
Presence of insufficient reductant within the SCR device to efficiently operate the SCR can have many causes. For instance, if a urea storage tank supplying urea to the injection system is empty, then insufficient reductant will be present. Another cause for insufficient reductant in the SCR device is contamination or dilution of the urea in the urea storage tank. If water is incorrectly added to the storage tank instead of urea, the efficiency of the SCR device is greatly reduced.
A number of different causes can result in reduced efficiency in an SCR device, including adverse properties in the exhaust gas flow, a malfunction or damaged catalyst in the SCR device, insufficient urea in the urea storage tank, and contaminated urea in the urea storage tank. A method to distinguish reduced efficiency in an SCR device based upon contaminated urea in the urea storage tank from other causes of reduced efficiency would be beneficial.